1. Field of the Invention
The present invention relates to a rotating body for a fluid dynamic bearing and a method of producing the rotating body for a fluid dynamic bearing, and in particular, to a technique in which the rotational accuracy of a rotating body can be secured while the rotating body is thinned.
2. Description of the Related Art
Electronic apparatuses recently deal with a huge amount of data, and accordingly rotating devices, such as hard disk drives (HDDs), are widely mounted therein as recording apparatuses. In a rotating device mounted in such an electronic apparatus, read/write of a huge amount of data is achieved by stably rotating a recording disk at high-speed. In order to make it possible, it is needed to stably rotate a rotating body in which the recording disk is mounted at high-speed. As the bearings by which high-speed rotation of a rotating body is made possible, fluid dynamic bearings are widely adopted. For example, in the fluid dynamic bearing disclosed in Japanese Patent Application Publication No. 2007-198555, the space between a sleeve of which part of a stator is composed and a shaft of which part of a rotating body is composed is filled with lubricant, thereby allowing smooth high-speed rotation of the rotating body to be achieved by the sleeve and the shaft being rotated in a substantially non-contact state with each other. Main parts of a rotating body in a rotating device in which a fluid dynamic bearing is adopted are a shaft that serves as a rotating shaft and a hub in which a recording disk is to be mounted. It is general that the shaft and the hub are produced as separate parts, taking into consideration a reduction in production cost and easy production thereof, and they are joined together by a method of press-fitting and adhesion, etc., in an assembly process.
There is a demand for miniaturization of electronic apparatuses in which rotating devices are mounted as well as improvement in performance. Also, the rotating devices in which fluid dynamic bearings are adopted are mostly requested to be miniaturized, in particular, to be thinned in the axial direction of the rotating shaft. Herein, the case where the thinning in the axial direction of the rotating shaft of the rotating device having a structure in which a shaft and a hub are connected together in an assembly process as stated above is to be achieved, will be considered. If the length of a shaft to be housed in a sleeve of which part of a fixed body is composed is made small for the thinning, the stability of supporting the shaft is deteriorated, and hence there arises the fear that eccentricity during rotation may be caused. Alternatively, thinning of the hub can be considered for the thinning of the whole rotating device; however, the thinning thereof induces a decrease in the stiffness as a hub as well as shortening of the axial connection margin occurring when the hub is connected to the shaft. The shortening of the connection margin induces a decrease in the connection strength between the shaft and the hub and a decrease in the squareness occurring when the two parts are connected together, and hence there arises the problem that stable high-speed rotation cannot be obtained as a rotating device for a fluid dynamic nearing. Alternatively, it can be considered that only the part of the hub corresponding to a connection margin is to be extended in the axial direction of the shaft; however, the length of the shaft to be housed in the sleeve is shortened by the length equal to the extended connection margin, and hence the same problems arise as in the aforementioned shortening of the shaft.